`theme(plot_title="blank")` doesn't work with gggrid

[IKupriyanov-HORIS]

Code:

from lets_plot import *
LetsPlot.setup_html()

gggrid([
    ggplot() + geom_blank() + ggtitle("Blank Plot #1"),
    ggplot() + geom_blank() + ggtitle("Blank Plot #2")
]) \
    + ggtitle("Grid Plot") \
    + theme(plot_title="blank")

Actual result: None of the titles are hidden.

Expected result: At least the "Grid Plot" title should be hidden.

Before v4.6.0, all titles were hidden. See Out[5] for reference: https://nbviewer.org/github/JetBrains/lets-plot/blob/f2b1ecadfe981362ce553be67ab9e6be2094ac9e/docs/f-23e/gggrid_theme.ipynb